Analysis of HER2-low breast cancer in Aotearoa New Zealand: a nationwide retrospective cohort study

Aim: To perform the first national analysis of demographic and clinicopathological features associated with the HER2 positive, HER2-low and HER2-zero invasive breast cancers in New Zealand. The study will inform the proportion of women who benefit from new HER2-targeted antibody drug conjugate (ADC) therapies. Methods: Utilising data from Te Rehita Mate Utaetae (Breast Cancer Foundation NZ National Register), the study analysed data from women diagnosed with invasive breast cancer over a 21-year period. The HER2 status of tumours was classified into three categories: HER2-zero, -low, -positive. Results: From 2009-2021, 94% of women underwent HER2 testing, with 14% diagnosed with HER2-positive breast cancer. For advanced-stage disease, 38% formerly classified as HER2-negative were reclassified as HER2-low. Including HER2-positive breast cancers, this indicates 60% of women with advanced breast cancer would be eligible for the new HER2-directed ADCs (approximately 120 women per year). In future, these therapies may provide a targeted option for 40% of women with early-stage triple negative breast cancer now classified as HER2-low. Conclusion: The findings suggest a significant proportion of women with invasive breast cancer in New Zealand could benefit from new HER2-targeted treatments. There is a need to standardise HER2 testing to enhance personalised treatment and improve outcomes.